Optical waveform for use in a DWDM optical network and systems for generating and processing same

Abstract

An optical signal occupying one or more wavelengths. An optical data signal on each wavelength is modulated with a respective overhead (dither) signal, resulting in a respective dithered optical signal. The amplitude of a particular overhead signal used to modulate the corresponding optical data signal is chosen so that the RMS value of the overhead signal in the dithered optical signal is proportional to the average intensity of the optical data signal. The instantaneous frequency of each overhead signal is time-varying and each possible frequency belongs to a distinct set of frequencies which are all harmonically related to a fundamental frequency. The distinctness of each set of frequencies allows each overhead signal to be uniquely isolated from an aggregate overhead signal. The harmonic relationship among the frequencies allows improved accuracy of RMS detection at a receiver as well as reduced computational complexity, as each possible frequency for each overhead signal can be made to fall at the center of one of the frequency bins of a single FFT of reasonable size performed at a receiver. Methods and systems for generating and detecting such signals are disclosed.

1. A method of determining the average intensity of at least one optical data signal, each optical data signal occupying a respective wavelength of interest in a received optical signal and having an intensity that is amplitude modulated by a respective overhead signal having an instantaneous frequency content that varies with time, the method comprising:
 (1) transforming the received optical signal into an aggregate signal in electrical form; 
 (2) transforming the aggregate signal into a frequency domain vector; 
 (3) for each wavelength of interest:
 (a) correlating the frequency-domain vector with a plurality of harmonically related frequency-domain templates to produce plural correlation results, said templates being uniquely associated with the wavelength of interest; 
 (b) processing the plural correlation results to produce a candidate frequency and a candidate RMS value; and 
 (c) validating the candidate frequency and the candidate RMS value. 
 
 
     
     
       2. A method as claimed in  claim 1 , wherein the step of transforming the aggregate signal into a frequency-domain vector comprises:
 sampling the aggregate electrical signal; and 
 executing a fast Fourier transform (FFT) on a finite number of samples, to produce the frequency-domain vector. 
 
     
     
       3. A method as claimed in  claim 2 , further comprising, between the steps of sampling and executing an FFT:
 acquiring the samples over an integer number of blocks each having a length substantially equal to the inverse of the fundamental frequency; 
 summing each block of windowed samples on an element-by-element basis to produce a block of summed elements; and 
 dividing each summed element by said integer number. 
 
     
     
       4. A method as claimed in  claim 3 , further comprising windowing the acquired samples prior to the step of summing. 
     
     
       5. A method as claimed in  claim 1 , wherein the step of processing comprises, for each wavelength of interest, performing an inverse fast Fourier transform (IFFT) on each correlation result, performing a peak detection on the result of each IFFT and performing a maximum detection on the set of peak detection results associated with the wavelength of interest. 
     
     
       6. A method as claimed in  claim 1 , wherein the step of validating comprises verifying whether the candidate RMS value at the candidate frequency is above a pre-determined threshold. 
     
     
       7. A method as claimed in  claim 1 , further comprising estimating the average intensity of the optical data signal at each wavelength from the respective validated candidate RMS value. 
     
     
       8. A method as claimed in  claim 7 , wherein estimating comprises dividing the respective validated candidate RMS value by a respective reference modulation depth. 
     
     
       9. A method as claimed in  claim 1 , futher comprising decoding control information from the value of the validated candidate frequency. 
     
     
       10. A method as claimed in  claim 1 , further comprising decoding control information from variations in the value of the validated candidate frequency. 
     
     
       11. A system for determining the average intensity of at least one optical data signal, each optical data signal occupying a respective wavelength of interest in a recieved optical signal and having an intesity that is amplitude modulated by a respective overhead signal having an instantaneous frequency content that varies in time, the system comprising:
 an opto-electronic conversion unit, for converting the recieved optical signal into a time-domain electrical signal;
 a frequency domain transformation unit connected to the opto-electronic conversion unit, for transforming portions of the time-domain electical signal into respective frequency-domain block; 
 for each wavelength, a plurality of matched filters connected to the FFT module, wherein each matched filter is adapted to correlate frequency-domain blocks recieved from the frequency domain transformatin unit with a unique template representative of frequency content which is harmonically related to a common fundamental frequency; 
 for each wavelength , a candidate selection unit connected to the matched filters associated with that wavelength, wherein each candidate selection unit is adapted to produce one canidate frequency and one candidated RMS; and 
 for each wavelength, a validation unit connected to the candidate selection units associated with that wavelength, for determining whether the candidate frequency and the cadidate RMS value are valid. 
 
 
     
     
       12. A system for determining the average intensity of at least one optical data signal, each optical data signal occupying a respective wavelength of interest in a received optical signal and having an intensity that is amplitude modulated by a respective overhead signal having an instantaneous frequency content that varies with time, the system comprising:
 means for transforming the received optical signal into an aggregate signal in electrical form; 
 means, connected to the transforming means, for transforming the aggregate signal into a frequency domain vector; 
 means, connected to the second transforming means, for correlating, for each wavelength of interest, the frequency-domain vector with a plurality of harmonically related frequency-domain templates to produce plural correlation results, said templates being uniquely associated with the wavelength of interest; 
 means, connected to the correlating means, for processing, for each wavelength of interest, the plural correlation results to produce a candidate frequency and a candidate RMS value; and 
 means, connected to the processing means, for validating, for each wavelength of interest, the candidate frequency and the candidate RMS value.